Remarks on the Enquiries ef Dr. Herfchel re/peeling Light and Heat. 419 



nately, gained an equal increafe of heat ; but, for ah obvious reafon, required to that effedl 

 double the time. The obfervation was next repeated by placing both thermometers in a 

 fituation to be equally affefted by the fcreen, the one in the focus of the lens and the other 

 belide it. The thermometers kept pace together in their progrefs, and their fludtuatlons, 

 as often as the fcreen was interpofed or withdrawn, correfponded with tolerable precifion. 

 The irregular difference of perhaps a quarter of a degree is furely a quantity too fmall to 

 form the ground of any legitimate inference. Yet mark with what confidence Dr. Herfchel 

 pronounces his precipitate conclufion : " All which fo clearly confirm the effedl of the rc- 

 " fra£l:ion of the lens, that it muft now be evident that there are rays ifluing from hot iron, 

 -•*,' which*, though in a Hate of total invifibility, have a, power of occafioning heat, and obey 

 " certain laws of refra£lion, very nearly the fame with thofe that affedt light." It is truely 

 aftonifliing to obferve how the mind, when once occupied by fome favourite idea, recalls it 

 at every ftep, and tortures every flight appearance into an argument for its fupport. 



In another paper fmce read at the Royal Society, Dr Herfchel purfucs his fubjed with 

 the fame monotonous prolixity. It chiefly confifts of experiments diredled to afcertain the 

 quantities of light tranfmitted through different coloured glafTes. But fuch refearches arc 

 of no real utility in a philofophical view, unlefs many other points had been fixed which 

 the Doftor entirely overlooks. The thicknefs of the glafs, its compofition, and the in- 

 tenfity of its Ihade, were of the moft eflential confequence. To determine that depth of 

 tranfmitted colour, would have required a comparifon with the triangular cells of Mayer, 

 as improved by Lambert, Achard, and Burja. The variable force of the fun's rays ought 

 alfo to enter into- the account, whether as afFefted by his different altitude above the 

 horizon, or by the condition of our atmofphere, and principally in refpedt to humidity. 

 I have obferved a very fenfible difference in the power of folar light in confecutive days at 

 noon, even when the fky was apparently clear. If colours offend the eye by excefs oc 

 defed, fome grofs eflimatc may be made of their refpedlive degrees of illumination; but 

 to determine thofe in general and with precifion by ocular contrail, feems altogether an im- 

 pradlicable, if not an abfurd, attempt. In facl, what juft comparifon can obtain, fqr 

 example, between green and red, which are flri£lly things as heterogeneous as tafte and 

 fmell. To increafe the embaraffment, we cannot always judge of the colour of the tranf- 

 mitted light by that of the fubftance through which the rays are fent. Thus a fun-beam, 

 in emerging from pafteboard, ivory, or white enamel, will, according to their thicknefs, 

 exhibit all the tints from yellow to deep red. Yet admitting, in its fulleft extent, the ac- 

 accusacy of Dr. Herfchel's refults, they furnifh no evidence whatever in fupport of his 

 hypothecs, or indeed of any other hypothefis. 



"^rhere is only a fingle paflage in the concluding paper that requires particular notice. 

 Dr. Herfchel had anticipated one confequence which I urged as an infurmountable objec- 

 tion to his fyftem; namely, that a burning glafs would a£l: moft fiercely at fome diftancc 

 behind the bright focus. The DoiSlor tried this, and, as he afferts, found it to fucceed ; 

 though he acknowledges at the fame time, and with great truth, that the experiment was 



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